Mantle Fluid Compositions Buffered in Peridotite-CO_2-H_2O by Carbonates, Amphibole, and Phlogopite

Abstract

Experimental and theoretical data from systems peridotite- CO_2-H_2O and CaO-MgO-SiO_2-CO_2-H_2O are combined, and extrapolated to 70 Kb for construction of partly schematic diagrams for subsolidus and near-solidus phase relationships of peridotite containing CO_2 and H_2O. The divariant solidus surface for peridotite-vapor in the presence of CO_2+H_2O is traversed by a series of univariant lines marking the intersections of divariant subsolidus carbonation/decarbonation and hydration/dehydration reactions occurring in the presence of CO_2-H_2O vapors. For H_2O-CO_2 contents up to certain limits, the vapor phase composition along these lines is buffered by carbonates or hydrous minerals, and liquid compositions are similarly defined. The approximate positions have been estimated for vapor-buffer lines on the solidus involving dolomite, amphibole, amphibole-dolomite, phlogopite, and phlogopite-dolomite. The buffering capacity of carbonate is far greater than that of hydrous minerals. Considering normal mantle peridotite with olivine, orthopyroxene and clinopyroxene, the maximum amounts of phlogopite and amphibole are produced by about 0.02% H_2O and 0.4% H_2O, respectively. About 5% CO_2 is required to produce the maximum amount of dolomite without complete reaction of clinopyroxene and loss of peridotite mineralogy. The buffered curve for partly carbonated peridotite extends to lower temperatures and higher pressures from an invariant point near 26 Kb and 1,200°C Near this line there is a temperature-maximum on the peridotite-vapor solidus. On the high-pressure side of this maximum, CO_2/H_2O is greater in liquid than in vapor; on the low-pressure side of the maximum (including all pressures below 26 Kb), CO_2/H_2O is greater in vapor than in liquid. Because of this maximum, near-solidus magmas rising along an adiabat would evolve dissolved volatile components in the depth range 100-80 km; this could contribute to explosive eruptions. At pressures greater than 30 Kb, mantle peridotite with H_2O and CO_2 melts along curves with vapor buffered to high CO_2/H_2O by dolomite (magnesite at higher pressures) or dolomite + phlogopite, or a vapor-absent curve for dolomite-phlogopite-peridotite, producing low-SiO_2 magmas. The relationships among carbon, carbonate, and oxygen fugacity are important for determination of magma compositions.